Optical devices, such as phase difference films and circularly polarized light controlling optical devices, are used, for example, in image display devices. In use, the optical device is in some cases incorporated in image display devices such as liquid crystal display devices. In the production of these image display devices, the assembly is sometimes heated at 200° C. or above, for example, for the formation of a polyimide film used as an aligning film or for the formation of an ITO film as a transparent electrode. Further, when the image display device using the optical device is used as a display in the inside of a car, the image display device is exposed to sunlight and consequently is sometimes heated by the sunlight to 100° C. or above. Therefore, the optical device, such as the phase difference film, used in image display devices such as liquid crystal display devices is possibly heated to a temperature of 100° C. or above and in some cases to 200° C. or above, depending upon the order of the incorporation of the optical device or the place where the Optical device is used.
On the other hand, in recent years, as described, for example, in Japanese Patent Laid-Open No. 100045/2001 and Published Japanese Translation of PCT Publication No. 508882/1998, Optical devices produced by polymerizing a polymerizable liquid crystal material have been proposed. These optical devices are advantageous in that properties of a liquid crystal can be anchored or immobilized by polymerization and can be used as a film. This has led to expectation of the development of optical devices to various applications.
Regarding optical devices such as phase difference films, Japanese Patent Laid-Open No. 2109/1993 discloses a stretched phase difference film possessing excellent heat resistance. Japanese Patent Laid-Open No. 142510/1993 discloses an optical device comprising a thermally polymerizable liquid crystalline polymer possessing excellent heat resistance. Further, Japanese Patent Laid-Open No. 133628/2001 discloses a highly heat-resistant polarizing diffractive film comprising a liquid crystal material which contains a polymeric liquid crystal and a crosslinkable material.
The optical device produced by polymerizing the polymerizable liquid crystal material, however, suffers from a problem that, for example, in the case of a cholesteric layer having cholesteric regularity, upon heating, a shift in center reflection wavelength disadvantageously occurs. Therefore, as described above, in use, when the optical device is incorporated in an image display device such as a liquid crystal display device, disadvantageously, the optical device can be used only in a site not exposed to heat during the production of the image display device.
Further, the stretched phase difference film disadvantageously undergoes a change in phase difference level at 80° C. or above, particularly at 100° C. or above, and, thus, when used, for example, in on-vehicle LCDs, poses an uneven display or other problems.